There is at present no effective form of chemotherapy available to humans affected with the major types of hereditary muscular dystrophy. In this connection the Line 413 dystrophic chicken is being employed as an animal model in which to test specific antiserotoninergic drugs either alone or in combination with drugs with an alternate mode of action of physical exercise. Primary criteria for measuring drug-related improvements include: a functional test to quantitate the onset and development of muscle weakness, blood enzyme and metabolite assays, and light and electron microscopic analyses of affected muscles. In the dystrophic chicken a positive correlation has been found between the appearance of abnormally high levels of blood serotonin, the onset of physical disability and the significant increases in plasma creatine-phosphokinase (CPK) activities. In this connection cinanserin and methysergide and other drugs antagonistic to serotonin have been found in the dystrophic chicken to delay significantly the onset in muscle weakness and in some cases markedly reduced the characteristically high levels of CPK. The possible cytotoxic effects of exogenous serotonin on in vitro and in vivo chicken muscle protein turnover will also be investigated.